The present invention relates to a valve and a manufacturing method thereof and, more in particular, it relates to a material used for a valve seat and a method of forming the valve seat. The valve according to this invention can be used in fluid machines such as turbines, pumps or blowers, internal combustion engines, chemical plants or nuclear power plants.
Valves are required to cause less abrasion and less seizure. It is also necessary that they cause less clinging and less erosion. It is preferred that they are also highly corrosion resistant. For safety relief valves, globe valves, gate valves, check valves, control valves, relief valves or butterfly valves used for various fluid machines, chemical plants or nuclear power plants, those having cobalt alloys referred to as stellite build-up welded to valve seats have been used so far. Further, since there is a worry for the exhaustion of cobalt resources and nuclear power plants are required to have cobalt free constitution, so that development of valve seat materials not containing cobalt has been demanded.
Japanese Patent Laid-Open No. Hei 11-63251 discloses use of an alloy formed by dispersing chromium boride particles to a nickel base alloy containing chromium, boron, and silicon as a valve seat material, which is build-up welded to a valve disc or a valve body. Further, Japanese Patent Laid-Open No. Sho 62-1837 and U.S. Pat. specification No. 4,754,950 disclose valves in which a valve seat for one of valve disc and valve body are formed of chromium-nickel-iron series iron-based precipitation hardened type alloy and the valve seat for the other of them is formed of a nickel base alloy having a hardness (Hv) of 400 or more. It is described that the nickel base alloy having a hardness (Hv) of 400 or more contains carbon, silicon, boron and chromium. It is also described that the nickel base alloy is bonded to the valve body or the valve disc by build-up welding, brazing or diffusion bonding. However, what is shown concretely is only those bonded by build-up welding.
The inventors of this invention have confirmed that a nickel base alloy in which one or both of silicide particles and boride particles are finely dispersed in a matrix of a metal microstructure has a ductility, is highly resistant to thermal shock or mechanical impact, excellent in wear resistance and seizure resistance and suitable to the use for valve seats. However, in a case of build-up welding the nickel base alloy in which silicide particles or boride particles are dispersed to the valve seat, it has been found that the nickel base alloy is melted upon welding to form a coarse dendritic solidified microstructure containing suicides or borides. The nickel-base alloy having the dendritic solidified microstructure is poor in the ductility, sensible to thermal shock or mechanical impact, and liable to be fractured.
This invention intends to provide a method of manufacturing a valve capable of maintaining a state where silicide particles or boride particles are finely dispersed without forming the dendritic solidified microstructure even in a case of using a nickel base alloy in which at least one of silicide particles and boride particles is dispersed in the matrix of a metal microstructure as a valve seat, and bonding the same to a valve disc or a valve body, as well as a valve obtained by the manufacturing method.